1. Field of the Invention
The present invention relates to a controller for a synchronous machine.
2. Description of the Related Art
In general, in a controller for a synchronous machine (an electric motor, a generator), a detector which detects a rotation phase angle of a rotor is required in order to perform a drive control for the synchronous machine. However, problems as will be described below are inherent in the controller using the detector. First, the presence of the detector increases a volume of the synchronous machine. Thus, expansion of an output of the synchronous machine is inhibited. Second, maintenance/inspection work for the detector itself becomes necessary. Thus, maintenance/inspection efficiency is deteriorated. Third, disturbance occurs on a detected value because of superposition of noise and the like on a signal line from the detector, and control performance is deteriorated. Fourth, most of the detectors require power supplies for driving themselves, and it is necessary to provide the power supplies belonging to other systems than those for synchronous machine drive sources. This causes an increase of loads on a power supply installation space, a power supply line, cost, and the like.
In consideration of the problems as described above, in recent years, there has been developed a control method of assuming the rotation phase angle without using the detector, and performing the drive control based on the assumed rotation phase angle. This method is called a “sensorless control”.
As a controller for the synchronous machine, which includes such sensorless controlling means, for example, in Japanese Patent Laid-Open Publication No. H8-205578 (published in 1996)), there is disclosed a technology, in which, in a system of driving the synchronous machine by a pulse width modulation (hereinafter, referred to as PWM) inverter, a variation of an output current generated by a PWM control and a variation of a time integration value of an output voltage are detected in synchronization with switching of the PWM, and the rotation phase angle is detected based on a current/voltage equation of the synchronous machine by using detected values thereof.
However, in the controller for the synchronous machine, which is as described above, since the rotation phase angle of the synchronous machine is assumed by using high-frequency components of the voltage and the current which are generated by the PWM, it is necessary to measure the current and the voltage at an arbitrary detection point, or to measure the current and the voltage by high-frequency sampling (such sampling will be referred to as high-speed sampling) of which frequency is higher than a switching frequency of the PWM. Heretofore, with regard to such current and voltage, input voltages obtained from individual measuring instruments have been converted into digital signals by an analog/digital converter (hereinafter, referred to as an AD converter), and a control operation has been performed therefor by using the digital signals. However, a special circuit and an expensive AD converter have been required in order to perform the measurement at the arbitrary point and the measurement using the high-speed sampling.